Retaining wall backfill

[pond68]

I'm in the process of getting a retaining wall poured. How long will I have to wait to have the contraction performed backfilling? And at what minimum % strength should the wall be at that the time of backfilling?

 

[msquared48]

The information you seek should have been spec'd in the structural notes for the wall by the designing engineer.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[vandede427]

the norm is 70-75%, but ultimately what Mike said is correct.

 

[pond68]

I'm the design engineer. Should I spec 75% as vandede427 mentionned?.

 

[cvg]

depends on information you have not provided. For instance is this retaining wall holding cut or fill? Is this a gravity or cantilever wall? How tall is the wall? Is it supporting a building or just landscaping? Are you backfilling with gravel or clay or flowable fill? What type of compaction are you asking for the backfill? Are you concerned with minor cracking of the wall or not?

 

[pond68]

It's a cantilever wall, 150 ft long & 9 ft tall. There is a building nearby on clay type soil. Backfill will be gravel, compaction by hand & cracking is a concern.

 

[ToadJones]

If you want to be really conservative, tell them not to backfill the wall, ever

 

[pond68]

Typically that's what I always recommend.

 

[cvg]

to get 75% with normal concrete will require somewhere between 7 - 14 days. Most contractors will not want to wait that long. Let them use a high early strength mix design or increase the strength of the mix so you can get adequate strength at 3 - 7 days.

Contrary to MMC, I never put the minimum strength or time on plans or specs, instead put the responsibility on the contractor to not damage the wall during backfill. Our local standards require 100% of the design strength for retaining walls.

 

[pond68]

Usually i don't put it either & recommend 100%. However no high early strength will be used. The contractor would like to proceed as early as possible but willing to wait 7 days.

 

[Zambo]

I would have thought 70% which should be achievable after 7 days. I don't think high early strength is required, but instead the contractor could just increase the concrete mix design by 1 grade, so for example instead of using 30MPa concrete use 35MPA.

 

[pond68]

Thanks all for the suggestions.

 

[asixth]

I am also interested in this post, I recently design a 8' high blockwall which is designed to be restrained at the top by a slab.

Construction sequencing I have put a note on the drawing which says that the blockwall is to remained propped until the slab over and blockwall have achieved design strength (I know the image is poor quality...It is a scan).

I have had a tough time with the contractor who looked at the note and says to me "What rating are the props?...What centres are the props at?...What anchors are required for the props?...If the props are at 1200 centres (metric), what timber support members are required to span between the props?".

I ended up saying that the props are required to be rated for 2.5tonne and 1200 centres with no timber members required to span between the props because the blockwall will have achieved sufficient strength to span horizontally prior to backfilling.

When a note says the wall is to be propped, should a rated load be given for the props and should a minimum design strength be given before backfilling the wall?

I have called up 20MPa grout (2900psi).

 

[mmillerpe]

asixth:

Typically where I work, the temporary bracing, or props fall within the means and methods of construction, and would generally fall under the contractors responsibility. Therefore the contractor should hire his own engineer to determine the required load rating, spacing, etc.

I would indicate such, and if he wanted to could hire you to design the temporary bracing system as an additional service.

 

[OzEng80]

asixth

I often get the propping issue you are referring to and normally palm it off similar to mmillerpe - with varying levels of success. In your instance you need far less than the 28day strength of the slab for it to function as a prop. If damage was to occur to the wall it would have happened during the cantilever/& backfilling stage (assume they did prop it for this). In this situation i prefer to design these types of walls as partial cantilevers (enough to take the earth pressure only) ie make your starters longer and place them on the 'earth side'. This provides a more robust design (though i still keep all the notes).

 

[vandede427]

don't forget your control joints and weep holes.